If your not going to be switching between the two platform frequently, I would recommend repartitioning your hard drive after you get your machine, and installing Windows in one partition and Linux in the other. Doing things that way is usually simpler, in that you don't need the over head of the VMs.
I really like using VMs for development, because it makes it really easy to maintain different configurations, make backups, test comms between machines, experiment, and so forth.
Linux VMs work pretty well. Windows in a VM on Windows, however, can be a resource hog. You probably want more than 4 GB on the laptop.
Sounds like you will get 15 minutes out of this laptop's battery, maybe 20.
Speaking from experience, you will prefer a desktop plus a "more mobile" laptop. You can do this without spending more than you had budgeted (remember you can skip the monitor on the deskop), but that will probably get you slightly less specs in exchange for the flexibility and a laptop you really can take with you. But I recommend spending slightly more than you would on the single laptop, and remember you do get two machines out of it.
You can network between them (e.g. use remote desktop programs from the laptop to connect to VMs on the desktop).
In my particular case, about 6 years ago I needed a new machine that could be used for on-location photography and had the power, screen-size, disk space, etc. to run Photoshop and other tools (e.g. batch processing ~900 largish images was one use case). I got a beefy laptop, which worked great for that, but the battery quickly died and never had much lifetime in the first place. The system has always been more of a "slightly-easier-to-move desktop" than a laptop, and it sounds like you'd rather have a real laptop.